The broad aim of this proposal is to determine the factors responsible for maintaining head position during motions of the trunk. The ability to stabilize the head is essential for maintaining gaze and postural orientation of the body during normal daily activities. Parcelling out the mechanisms responsible for head stability should assist in evaluating and treating the causes of gaze and balance disorders. Specifically, four mechanisms involved in head stabilization will be characterized. They include: the biomechanical parameters of the head-neck motor system, the vestibulocollic and cervicocollic reflexes, triggered reactions, and voluntary reaction-time movements. To accomplish these goals, both normal and bilateral labyrinthectomized patients will be tested in a multifactorial paradigm. Subjects will be rotated in the yaw, pitch, and roll planes in both the seated and standing positions to measure head stability about the center of mass of the head and the center of mass of the body. Five experimental conditions are designed to differentiate between the four head stabilizing mechanisms. The first will use mental arithmetic to distract the subject in order to eliminate voluntary inputs and emphasize the effects of the biomechanical and reflex components. Next, the subject will stabilize his head while blindfolded so that voluntary responses are added to the vestibular and proprioceptive inputs. During the next condition, the subject will have visual cues to assist in head stabilization. Then the effectiveness of visual cues will be assessed when the subject tracks a moving target. Finally, the subject will be rotated with the head fixed to the chair so that neck proprioceptive inputs are eliminated and vestibular and head inertial effects can be measured in an open loop mode. Two stimulus waveforms will be used. A random sum-of-sines rotation will produce high fidelity frequency responses. Pseudorandom binary sequence stimuli will be used to obtain time domain responses that reveal latencies and amplitudes of various response components. Both electromyographic activity of neck muscles, and recordings of head movement and torques will be recorded during the five conditions described above. Measures of gains, phase, and latencies, as well as the magnitude of the inertial and reflex torques will be used to construct models of the observed behaviors. Quantitative identification of the mechanisms involved in head stabilization within this multifactorial design should result in the first complete 3-dimensional analysis of the processes underlying head stabilizatyion. The studies proposed here should result in a clearer delineation fo the four mechanisms underlying most purposive movements. Beginning analyses in the area of vertical head movement control is an important step toward identifying the correspondence between studies of gaze, head, and total body stability.